Technical Field
The present invention relates to an outer package material for a lithium-ion battery and a method for producing a lithium-ion battery using the outer package material.
Background Art
Nickel hydride and lead storage batteries are known to be used as secondary batteries. The downsizing of secondary batteries, however, is required because of the downsizing of mobile devices, limitations of installation space, etc. Accordingly, attention is being paid to a lithium-ion battery having high energy density. As an outer package material (referred to as an “outer package material” herein) to use in a lithium-ion battery, although a metallic can was used conventionally, a multiple-layered film has been used that is light, has high radiation performance and can be applied at low cost.
An electrolytic solution of a lithium-ion battery contains an aprotic solvent such as propylene carbonate, ethylene carbonate, diethyl carbonate or ethylmethyl carbonate, and an electrolyte. As a lithium salt which is an electrolyte, lithium salts such as LiPF6 or LiBF4 are used. These lithium salts, however, generate hydrofluoric acid because of a hydrolysis reaction with water. Hydrofluoric acid causes corrosion on a metallic surface of a battery member or decreases the laminate bond strength between layers of the outer package material made of a multiple-layered film.
Accordingly, an aluminum foil layer is provided inside an outer package material made of a multiple-layered film, which prevents water from entering through the surface of the multiple-layered film. For example, an outer material wherein a base layer having heat-resistance/a first adhesive layer/an aluminum foil layer/a corrosion prevention treatment layer which prevents corrosion due to hydrofluoric acid/a second adhesive layer/a sealant layer are sequentially layered is known. The lithium-ion battery where the above-described outer package material is used is called a lithium-ion battery of aluminum laminate type.
As the lithium-ion battery of aluminum laminate type, a lithium-ion battery of embossed type is known. In this lithium-ion battery of embossed type, a recess is formed on a part of the outer package material by cold molding, battery contents (a positive electrode, a separator, a negative electrode, an electrolytic solution, etc.) are accommodated in the recess, the rest of the outer package material is folded, and edge portions are sealed by heat-sealing. Recently, in order to increase energy density, a lithium-ion battery where recesses are formed on both two outer package materials to be stuck together has been produced. This type of lithium-ion battery can accommodate more battery contents.
The energy density of the lithium-ion battery is increased as the depth of the recess formed by cold molding is increased. However, a pinhole or breaking readily occurs at the outer package material in molding as the depth of the formed recess is increased. Accordingly, biaxially-stretched polyamide film has been used for the base layer of the outer package material to protect the metallic foil.
Here, as an example for improving moldability, it has been suggested that a film can be used wherein the film's tensile strengths in four directions, 0°, 45°, 90° and 135°, until occurrence of any breaking in a tensile test are 150 MPa, and elongations in the four directions are 80% or more (for example, see PTL 1).